In producing high carbon steel wire, the normal practice is to patent and draw a hot rolled wire rod one or more times to finish it to a predetermined wire diameter. This high carbon steel wire has to be ensured a predetermined strength and be ensured a performance sufficient even for the toughness/ductility evaluated by the drawing ratio at break etc.
The fact that for increasing the strength of high carbon steel wire, increasing the amount of C in chemical compositions of the steel is the most economical and effective means has been confirmed. However, if the increase in the amount of C causes the steel material to become a hyper-eutectoid composition, at the time of rolling or patenting, when cooling from the austenite region, proeutectoid cementite tends to precipitate in a network at the austenite grain boundaries. This tendency appears more remarkably when there is center segregation of C at the center of the wire rod. Further, at the high hardenability center segregation part, micromartensite tends to be formed. As a result, the frequency of breakage at the time of wire drawing also becomes high, thereby inviting a drop in productivity or yield and resulting in poor toughness/ductility of the wire after drawing.
Therefore, Japanese Unexamined Patent Publication (Kokai) No. 2002-129223 proposes a method of including in molten steel with solidified primary crystals of γ-Fe 1 to 10 μm inclusions in an amount of 1 to 500/mm2 to obtain a bloom or billet having a fine solidified structure and using this bloom or billet to produce high carbon steel wire. Further, Japanese Unexamined Patent Publication (Kokai) No. 2001-64753 proposes, for the purpose of improving the lubrication performance in a high carbon steel wire rod for large diameter of steel wire, making the oxide-based inclusions containing Zr etc. hard inclusions of 70% or more of Al2O3 in composition. Further, Japanese Unexamined Patent Publication (Kokai) No. 2003-96544 proposes high carbon steel wire rod in which delamination is suppressed and ductility is improved by adding either or Mg or Zr to cause formation of fine oxides or sulfides and reduce the solid solution C after patenting.
Next, in producing the above-mentioned bloom or billet, molten steel with solute concentrated among the dendrites moves to the center of the bloom or billet due to the solidification contraction or the flow at the end of solidification due to roll bulging etc. resulting in center segregation. Further, due to the solidification contraction, porosity sometimes occurs at the center of the bloom or billet. In high carbon wire rod, C and Mn concentrate at the center segregation part, so proeutectoid cementite is formed at the austenite grain boundaries, micromartensite is produced, breakage is caused at the time of wire drawing, or the toughness after wire drawing becomes poor.
As the method of suppressing this center segregation, in continuous casting of blooms or billets, using electromagnetic stirring to cause the formation of equiaxed crystals is a widespread practice. In the case of solidification of columnar crystals, the center segregation occurs mostly at the bloom or billet center, but by using this method, the center segregation can be distributed among the equiaxed crystal grains. Further, in continuous casting, the method of reducing the bloom or billet by rolls by exactly the amount corresponding to the amount of solidification contraction at a position where the solid phase ratio of the center part becomes 0.3 to 0.7 so as to suppress flow of solidification contraction and prevent center segregation (soft reduction method) is well known.
Among these, electromagnetic stirring is a method of stirring at the further downstream side of the strand than the method of stirring in the mold, but for converting the solidified structure to equiaxed crystals, it is known that electromagnetic stirring in the mold is extremely effective. However, if performing electromagnetic stirring in the mold, the continuous casting powder becomes entrained and causes defects. For example, with high carbon wire rod, this sometimes becomes a cause of breakage at the time of wire drawing. Therefore, there is a limit to raising the thrust of the electromagnetic stirring in the mold. Further, equiaxed crystals obtained by electromagnetic stirring are relatively large equiaxed crystals, so there is the problem that the segregated grains at the center segregation (size of parts where the solute becomes remarkably concentrated near the center of the bloom or billet) do not become sufficiently fine.
On the other hand, with the soft reduction method, if the timing of reduction can be made suitable, an extremely great center segregation suppression effect can be obtained, but if the reduction is too early or too late, reverse V-segregation or V-segregation will occur. In general, there is a variation in the growth of a solidified shell in continuous casting. With just soft reduction, sometimes incomplete formation occurs.
In the above way, sufficient reduction of center segregation in continuous casting is an important technical issue even at the present.
As another method for suppressing such center segregation, there is the method of causing fine inclusions to distribute in molten steel and utilizing these as nuclei for the formation of heterogeneous nuclei at the time of solidification so as to raise the equiaxed crystal zone ratio and make the equiaxed crystals finer.
The above mentioned Japanese Unexamined Patent Publication (Kokai) No. 2002-129223 discloses a bloom or billet provided with a fine solidified structure characterized by including and causing solidification of inclusions with a lattice strain with γ-Fe of 7% or less in molten steel where the solidified primary crystals are γ-Fe. Further, as these inclusions, ones containing one or more of MgS, ZrO2, Ti2O3, CeO2, or Ce2O3 may be mentioned.